1. Field of the Invention
The present invention relates to an ink jet head and an ink jet recording apparatus applicable, for example, to a printer or a facsimile machine.
2. Description of the Related Art
An ink jet recording apparatus has been known which records characters and images on a recording medium by using an ink jet head having a plurality of nozzles for ejecting ink. In such an ink jet recording apparatus, the nozzles of the ink jet head are provided in a head holder so as to be opposed to the recording medium, and the head holder is mounted on a carriage, with which scanning is performed in a direction perpendicular to the direction in which the recording medium is transported.
FIG. 11 is an exploded perspective view of an example of such an ink jet head. As shown in FIG. 11, a plurality of grooves 132 are arranged side by side in a piezoelectric ceramic plate 131, and the grooves 132 are separated from each other by partition walls 133. One longitudinal end portion of each groove 132 extends to one end surface of the piezoelectric ceramic plate 131, whereas the other end portion thereof does not extend to the other end surface, and its depth gradually decreases. Further, on the opening side surfaces of the partition walls 133 on both sides of each groove 132, electrodes 134 for applying a drive electric field are formed so as to extend in the longitudinal direction.
Further, attached to the opening side of the grooves 132 of the piezoelectric ceramic plate 131 is an ink chamber plate 136 defining a common ink chamber 135 communicating with the shallow end portions of the grooves 132.
Further, fixed to the ink chamber plate 136 is a flow passage substrate 138 which seals one side of the common ink chamber 135 and which has a communication hole 137 communicating with an ink supply passage for supplying ink to the common ink chamber 135.
Provided in the flow passage substrate 138 is an ink sump 139 constituting a part of the ink flow passage for supplying ink from the communication hole 137 to the common ink chamber 135.
A nozzle plate 140 is attached to the end surface of the joint unit formed by the piezoelectric ceramic plate 131 and the ink chamber plate 136 where the grooves 132 are open, and nozzle openings 141 are formed at the positions of the nozzle plate 140 opposed to the grooves 132.
In this ink jet head, constructed as described above, the grooves 132 are filled with ink from the communication hole 137, and when a predetermined drive electric field is applied to the partition walls 133 on both sides of a predetermined groove 132 through the electrodes 134, the partition walls 133 are deformed to change the volume of the predetermined groove 132, whereby the ink in the groove 132 is ejected from the corresponding nozzle opening 141.
In this ink jet head, the diameter of the nozzle openings 141 is very small, so that even if the volume of each groove 132 is varied, it is impossible to eject ink from the nozzle openings 141 in a satisfactory manner unless back pressure is generated in each groove 132 from the common ink chamber 135 side.
In view of this, in the conventional ink jet head, a mesh-like filter 142 formed, for example, of stainless steel (SUS), is provided between the ink sump 139 and the common ink chamber 135. Due to this filter 142, back pressure is generated in the grooves 132 to eject ink droplets from the nozzle openings 141.
The ink sump of the conventional ink jet head, however, includes a region where ink does not flow easily. For example, in a region such as a corner of the ink sump, the ink supplied to the ink sump does not flow easily, with the result that bubbles stay in that region. When bubbles stay in such a region, the volume of the ink sump changes, resulting in a deficiency in the ink supply to the ink chamber. In particular, when, for example, an ink with poor bubble permeability such as a water-color ink is used, bubbles are likely to be generated in the ink, with the result that the deficiency in ink supply is gradually aggravated.
It is general practice to remove the bubbles thus accumulated in the ink sump through a so-called cleaning operation in which the bubbles are sucked from the nozzle opening side. However, even by this cleaning operation, it is difficult to substantially remove the bubbles staying in the portion of the ink sump on the upstream side of the filter, which do not pass through the filter.
In an ink jet head in which bubbles cannot be removed from a region where ink does not flow easily, bubbles staying in the ink sump may pass through the filter to be ejected together with ink during printing, thereby causing a printing failure. Thus, such an ink jet head is to be disposed of, resulting in a poor yield rate.
Further, when the size of the ink droplets is large or the number of nozzle openings is large, that is, when the amount of ink ejected per unit time is large, the amount of bubbles existing, in particular, in the ink sump, is large, so that the area of the ink flow passage is substantially reduced, resulting in the deficiency in the ink supply to the ink chamber being gradually aggravated.
It might be possible to accelerate the ink flow by narrowing the flow passage of the ink sump to thereby prevent the bubbles in the ink sump from staying. This, however, would involve a substantial reduction in the filter size, resulting in a deficiency in the ink supply to the common ink chamber. Further, even by adopting this method, it is impossible to completely remove the bubbles staying in the portion of the ink sump on the upstream side of the filter through the above-mentioned cleaning operation.
The present invention has been made in view of the above problem in the prior art. It is an object of the invention to provide an ink jet head and an ink jet recording apparatus in which it is possible to reliably prevent ink bubbles from staying in the ink sump and the interior of the head chip and in which it is relatively easy to remove the bubbles.
According to a first aspect of the present invention for solving the above-mentioned object, there is provided an ink jet head including: a plurality of grooves arranged side by side so as to communicate with nozzle openings; an ink chamber for supplying ink to each of the grooves; an ink sump provided so as to communicate with the ink chamber; and ink storage unit communicating with the ink sump through ink supply passages,
characterized in that there is provided an air duct establishing communication between a region in the ink sump which is substantially free from interference with an ink flow from the ink supply passages to the ink chamber and in which bubbles easily gather and an air region of the ink storage means.
According to a second aspect of the present invention, in the first aspect of the present invention, the ink jet head is characterized in that the ink chamber is provided so as to extend over the direction in which the grooves are arranged side by side, that the ink chamber and the ink sump communicate with each other over the direction in which the grooves are arranged side by side, that the ink supply passages are provided on both sides of the ink sump with respect to the direction in which the grooves are arranged side by side, and that the air duct is provided between the ink supply passages.
According to a third aspect of the present invention, in the first or second aspect of the present invention, the ink jet head is characterized in that the ink sump is equipped with a tapered portion gradually diverging from at least one of the openings of the ink supply passages and the air duct toward the ink chamber.
According to a fourth aspect of the present invention, in any one of the first to third aspects of the present invention, the ink jet head is characterized in that the ink sump is equipped with a filter situated in the vicinity of the border between the ink sump and the ink chamber.
According to a fifth aspect of the present invention, in the fourth aspect of the present invention, the ink jet head is characterized in that the air duct communicates with the portion of the ink flow passage which is on the upstream side of the filter of the ink sump.
According to a sixth aspect of the present invention, there is provided an ink jet recording apparatus equipped with an ink jet head according to any one of the first to fifth aspects.
According to the present invention, there is provided an air duct establishing communication between a region which is substantially free from interference with the ink flow in the ink sump from the ink supply passage to the ink chamber and in which bubbles gather easily and the air region of the ink storage unit, whereby it is possible to reliably prevent the bubbles in the ink from remaining in the ink sump and the interior of the head chip, and the bubbles can be removed relatively easily.